1. Field of the Invention
The present invention relates to a toner comprising a binder resin, a coloring agent and a releasing agent for developing latent electrostatic images, which is used in the field of electrophotography, electrostatic recording and electrostatic printing.
2. Discussion of Background
Generally, latent electrostatic images are formed on a photoconductor in the electrophotographic process, and they are formed on a dielectric material in the electrostatic recording process. When the latent electrostatic images thus formed on the latent-electrostatic-image-supporting-member are developed into visible images with a dry-type toner, the above-mentioned toner, which is in the form of finely-divided particles, is supplied to the surface of a toner-supply roller such as a development sleeve and uniformly distributed around the toner-supply roller by a toner-layer-thickness regulation blade to form a thin toner layer. While the toner is frictioned, it is positively or negatively charged. The toner is thus attracted by the latent electrostatic images formed on the latent-electrostatic-image-supporting member. The visible toner images thus formed are transferred onto a transfer material such as a sheet of paper when necessary, and fixed thereon by the application of heat and pressure thereto or by the application of a vaporized solvent.
For developing the latent electrostatic images formed on the latent-electrostatic-image-supporting member into visible images, there are conventionally proposed two methods; a wet-type developing method using a liquid type developer and a dry-type developing method using a dry-type developer, as previously noted. The dry-type developer includes a one-component dry-type developer comprising a toner and a two-component dry-type developer comprising a toner and a carrier.
When the two-component dry-type developer is employed, the development method varies depending upon the kind of carrier contained in the developer. For instance, when iron powder is used as the carrier, latent electrostatic images are developed by the magnetic brush development process. In the case where beads carrier is employed, cascade development process is performed. Furthermore, when the above-mentioned beads carrier is replaced by a fur brush, latent electrostatic images are developed by the fur brush development process.
On the other hand, in the case of the one-component dry-type developer, there are also many methods for developing latent electrostatic images For example;
(i) Powder cloud development: Development is performed by toner particles which are sprayed through a nozzle in the air.
(ii) Contact development (or toner development): Development is performed by physically bringing toner particles into contact with the latent electrostatic images.
(iii) Jumping development: Development is performed by charging toner particles to a predetermined polarity and causing them to jump at the latent electrostatic images having an electrical field.
(iv) Magne-dry development: Development is performed by bringing magnetic electroconductive toner particles into contact with the latent electrostatic images.
The conventional toners which are applicable to the aforementioned development methods comprise a low-molecular-weight polyethylene or polypropylene as a releasing agent to prevent the off-set phenomenon in the image fixing operation. Compatibility of this kind of releasing agent with a styrene-based binder resin is not so good that a releasing agent component and a binder resin component are separated while kneaded to prepare a toner composition. The interfaces between two components are easily broken and toner particles are thus finely ground when the mechanical force is applied thereto. Such a phenomenon occurs at the contact surface of the photoconductor and the transfer sheet, the contact surface of the development sleeve and the toner-layer-thickness regulation blade, and the contact surface of the development sleeve and the photoconductor.
In the case where the latent electrostatic images are developed by the aforementioned contact development process, the finely ground toner particles are deposited on the surface of the development sleeve and assume in the fused state with time. As a result, a so-called toner-filming phenomenon takes place. Because of this phenomenon, the thickness of a thin toner layer around the development sleeve becomes nonuniform and the charge quantity of the toner becomes uneven. This makes it impossible to constantly yield images with a high image density. This is a critical problem to the image quality.
To avoid the toner-filming phenomenon, therefore, a high-molecular-weight polymer is blended in the toner. However, in the case where the latent electrostatic images are developed into visible toner images with the toner comprising the high-molecular-weight polymer, it is required to raise an image fixing temperature while the toner images are fixed onto a transfer sheet with the application of heat thereto. Consequently, much thermal energy is required at the image fixing step, which has an adverse effect on the energy saving In addition, the size of a copying apparatus cannot be decreased.
Another proposal is made to avoid the toner-filming phenomenon. Namely, there is proposed a toner comprising a small amount of a plasticizer. This kind of toner does not necessarily succeed in preventing the toner-filming phenomenon. This is because the fluidity of the toner is decreased and the toner particles adhere to the carrier and the carrier is stained therewith (so-called spent-toner problem).
When the toner is too hard, on the contrary, it is difficult to mechanically crush the toner and various components in the toner composition cannot sufficiently be dispersed.
Under such circumstances, the conventional toner comprises as a binder resin a relatively low-molecular weight polystyrene or a styrene - butyl methacrylate copolymer which has an appropriate hardness. However, it is confirmed that the hardness of the above-mentioned relatively low-molecular weight polystyrene and styrene-butyl methacrylate copolymer is not sufficient when they are used in, for example, a laser printer, which is expected to be maintenance-free. In addition, this kind of binder resin is disadvantageous when the image is fixed onto a transfer sheet by using a heat-application roller. Specifically, although the adhesion of this kind of binder resin to a transfer sheet is good, it also sticks to a heat-application roller and causes the off-set phenomenon.
In order to prevent the above-mentioned toner-filming phenomenon on the development sleeve and the photoconductor, and solve the spent-toner problem, various proposals are further made. For example;
(1) A metallic soap is used as a fluidity-promoting agent in Japanese Laid-Open Patent Applications 47-36405 and 47-36830.
(2) A fluorine-containing compound is used as a fluidity-promoting agent in Japanese Laid-Open Patent Applications 52-153441 and 53-147541.
(3) A nonionic surface active agent is used as a fluidity-promoting agent in Japanese Laid-Open Patent Application 54-8534.
(4) Silica, the surface of which is treated to be hydrophobic is used as a fluidity-promoting agent in Japanese Laid-Open Patent Application 56-62256.
(5) Particles whose hardness is higher than that of a toner particle are embedded into the toner as in Japanese Laid-Open Patent Application 56-66856.
(6) An ion exchange resin is contained in the binder resin as in Japanese Laid-Open Patent Application 58-134651.
(7) A toner comprises an oxidized polyethylene as a releasing agent, which is compatible with the binder resin, as in Japanese Laid-Open Patent Application 59-131943.
(8) A silicone oil is contained in a binder resin as in Japanese Laid-Open Patent Application 56-197048
(9) Finely-divided particles of wax are attached to the surfaces of toner particles as in Japanese Laid-Open Patent Application 59-220748.
(10) Particles of carbon black are attached to the surfaces of toner particles to lower the resistivity of the toner as in Japanese Laid-Open Patent Application 60-138565
(11) Finely-divided particles of various polymers are attached to the surfaces of toner particles as in Japanese Laid-Open Patent Applications 60-186851, 60-186852, 60-186853, 60-186854, 60-186855, 60-186857, 60-186858, 60-186860, 60-186861, 60-186862, 60-186863, 60-186864, 60-186865 and 60-186866.
(12) Particles of an abrasive agent (SiC or SiN) are attached to the surfaces of toner particles as in Japanese Laid-Open Patent Application 61-99164.
The aforementioned additives do not successfully prevent the toner-filming phenomenon and solve the spent-toner problem.